The standard “ASTM G22-23 Standard Practice for Determining Resistance of Plastics to Bacteria” developed by the American Society for Testing and Materials (ASTM) describes two procedures for determining the effect of bacteria on the properties of plastics in the form of molded and fabricated articles, tubes, rods, sheets, and film materials.
Procedure B provides more extensive contact between the test bacteria and the samples than Procedure A. In Procedure A, the test is performed by placing the sample on the agar surface. In Procedure B, the sample is encapsulated in the agar, providing more extensive contact between the bacterial inoculum and the test material. Changes in optical, mechanical, and electrical properties can be determined by current ASTM methods. Environmental conditions such as high temperature and humidity can be favorable for bacterial growth. The procedures described allow testing of the resistance of plastic materials to bacteria when used in such situations.
The procedure described in this standard consists of the following steps:
Because these procedures involve handling and working with bacteria that can infect humans, the tests must be performed by personnel trained in microbiology.
The resin portion of plastic materials is generally resistant to bacteria because it does not serve as a carbon source for bacterial growth. Other ingredients such as plasticizers, lubricants, stabilizers, and colorants are often responsible for bacterial attack on plastic materials. When plastics are used in conditions of high temperature and humidity that are conducive to such attack, it is important to determine the resistance of plastics to microbial attack.
The expected effects are:
Generally, changes in electrical properties are primarily due to surface growth and associated moisture, and pH changes caused by the products of bacterial metabolism. Other effects include preferential growth resulting from non-uniform distribution of plasticizers, lubricants, and other processing additives. Significant physical changes may be observed in products in film form or coatings where the surface-to-volume ratio is high and nutrients such as plasticizers and lubricants continue to diffuse to the surface as they are used by the organisms.
Because the attack by organisms involves a large element of change due to local accelerations and inhibitions, the order of repeatability may be quite low. To ensure that predictions of behavior are not too optimistic, the largest degree of perturbation observed should be reported.
Conditioning of samples, such as exposure to leaching, weathering, heat treatment, and similar influences, can have significant effects on the resistance of plastics to bacteria. The determination of these effects is not addressed in this standard.
The glass containers used in these tests are suitable for holding the samples when laid flat. Depending on the size of the samples, the following are recommended:
The tests also use an incubator. For all test methods, the incubation equipment should provide a temperature between 35 degrees and 37 degrees and a relative humidity of at least 85 percent. Automatic recording of wet and dry bulb temperature is recommended.
Reagent grade chemicals should be used in all tests. Unless otherwise specified, all reagents should conform to the specifications of the American Chemical Society Committee on Analytical Reagents, where such standards are available. Other grades may be used provided that the reagent is found to be of sufficiently high purity to permit its use without diminishing the accuracy of the determination.
Unless otherwise stated, the water used in tests is distilled water.
Nutrient salts agar is a carbon-free culture medium used in the ASTM G21-15 standard. Although designed for growing fungi, nutrient salts agar supports the growth of test bacteria when the carbon requirements are met by sensitive plastic materials. This medium is prepared by dissolving the specified amounts of the reagents specified in the standard in 1 L of water.
The test medium is sterilized by autoclaving at 121 degrees for 20 minutes. Sufficient medium is prepared for the necessary tests, including unvaccinated controls.
Standard tests are performed against a bacterial species, Pseudomonas aeruginosa. The test sample is inoculated with the appropriate bacterial culture and incubated for 21 days. When applying the test methods, the inhibition of bacterial growth by the sample is assessed by visual assessment and confirmed by microscopic examination.
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